This invention relates to hydroforming dies and more particularly to hydroforming dies that provide for mechanically forming as well as hydroforming parts.
With the hydroforming dies and associated positioning apparatus commonly used to hydroform tubular metal parts to a large variety of different shapes, certain difficulties prone to produce scrap parts can arise in the hydroforming process. And typically result from the particular hydroforming die cavity configuration formed by the dies and/or the shape of the part as received by the dies. For example, very precise repeatable alignment of the dies during their closure on the part may be required to avoid having to scrap unacceptable misshaped parts following hydroforming such as can result when the part as received by the dies has a substantial interference configuration with respect to the die surface on either one or both of the dies as they close on the part.
Another difficulty apart from inaccurate die alignment that can arise resulting in scraped parts following their hydroforming is pinching of the part between the dies during die closure. And again because of the particular die cavity configuration and/or the shape of the part as received by the dies. Such instances have been found to mostly occur where the die cavity configuration will not accept, without forced engagement, a tubular part that is bent to a considerable degree and/or has a cross-sectional dimension considerably larger than that of the die cavity.
Relating these findings to prior known hydroforming dies and attendant apparatus such as disclosed in U.S Pat. Nos. 5,233,854 and 5,233,856 assigned to the assignee of this invention, such problems do not typically arise unless the particular die cavity configuration and/or the shape of the part as received by the dies forces these issues as mentioned above. In the latter event, there are various approaches to avoiding these problems. For example, the hydroforming dies may be guided into alignment, in a manner like steel stamping dies, by pins on one of the dies that are received in bushings on the other die. In that case, very close tolerances can be imposed with respect to the relative pin and bushing locations on the dies. And the tolerances between the pins and bushings can be reduced to a very close slip fit in the quest for accurate die alignment. But this still may not solve the problem because the peripheries of the die cavity surfaces on the dies are not a close enough match. In that case, a demand is also placed on forming the hydroforming die surfaces so that their peripheries are made to match as closely as possible with the best state-of-the-art die cutting machinery and tools.
Apart from die alignment and die cavity surface periphery matching, a pinching problem can still remain because of shape of the part as received by the dies. For example, the part may be bent as a result of improper handling or having been cut to the desired length from bent tubular stock as received from the tube stock manufacturer and/or it may have a cross-sectional shape or profile considerably larger than that of the die cavity. Two possible solutions are known to have been proposed to solving such problems and these are (1) prefilling the part with the hydroforming fluid at a certain pressure prior to closing the dies on the part, and (2) mechanically preforming the part to a certain degree in a set of preforming press dies. In the latter instance, the part is preformed so as to compensate for an otherwise unaccommodating region(s) of the hydroforming die cavity surfaces as the dies are pressed together about the part. However, these approaches to preventing pinching involve significant added costs. Moreover, while such approaches have proven generally satisfactory, there can still remain significant problems with certain die cavity configurations. And particularly with respect to pinching where even small dimensional deviations in the part (either preformed, filled or not preformed) can result in repeated die tryouts, scraped parts and subsequent die and/or part modifications before a satisfactory solution is found.
In the present invention, certain features are incorporated in the hydroforming dies that provide for precise die alignment and according to need, also provide for mechanical straightening and/or bending of the tubular part to be hydroformed and also mechanical reshaping of the cross-section of the part. With all such operations on the tubular part being performed by the hydroforming dies as they close on the part to effect precise repeatable die alignment and prevent pinching of the part prior to hydroforming the part in the dies.
In that certain features of the hydroforming dies of the present invention might standing alone be considered in a broad sense as jaw-like devices and considering the background of such devices, there are of course many various prior known devices of this general type using various forms of jaws or the like. But in form and function, they are quite different from the present invention as will be seen from both the summary and detailed description of invention that appears later herein. Examples of such prior jaw-like devices are disclosed in U.S. Pat. No. 468,272, which relates to a box clamp, U.S. Pat. No. 1,666,844, which relates to a nail straightening device, U.S. Pat. No. 3,251,216, which relates to an electrical connector applier, U.S. Pat. No. 4,283,933, which relates to a pair of pliers, and U.S. Pat. No. 4,599,921, which relates to a drill holder. These devices all grip or clamp a part but neither release the part for continued processing of the part in the device nor are they intended to or readily adaptable to doing so in a release mode.
The present invention diverges from previous known approaches to solving the above mentioned hydroforming die alignment and pinching problems by tackling these problems directly in design of the hydroforming dies rather than with attendant die alignment apparatus, reshaping the cross-section of the part in a preforming operation and/or prefilling the part prior to insertion of the part in the dies. This is accomplished with a significantly improved hydroforming die design that produces in a very cost-effective manner very precise and repeatable die alignment while positively preventing pinching of the part. And therefore eliminates or at least significantly diminishes the need for repeated die tryouts and the number of scrapped parts which is especially significant in a high volume production run of critical and costly parts; for example, the hydroforming of motor vehicle parts such as motor vehicle frame parts.
In the present invention, the dies for hydroforming a part are provided with die cavity forming surfaces on their inner side which cooperatively form a desired die cavity about a tubular part to be processed and also partially mechanically shape the part when the dies are pressed together in precise alignment. And wherein the die cavity thus formed has, as is conventional, openings open to the ends of the part for the supply of hydroforming fluid to the interior of the part and eventually the exhaust thereof. Such precise die alignment is provided by fingers on the inner side of each of the dies that are closely received in and guided by slots in the inner side of the other die as the dies are brought together. And wherein the fingers and slots on the inner side of each die are arranged in rows on opposite sides of and along the periphery of their respective die cavity forming surface and extend transverse to their respective die cavity surface periphery.
The die fingers are separated by the slots in each row and are staggered with respect to those in the other row on the respective die. In addition, the fingers have a flat tubular part engaging edge surface and the edge surfaces of the fingers in the two rows on each of the dies face oppositely and extend at an angle to their respective die cavity forming surface. The edge surfaces of the fingers in the two rows on each of the dies form an acute angle there-between such that as the dies are pressed together about a tubular part positioned between the fingers on the dies, these edge surfaces of the fingers cooperate to produce a funneling wedging scissoring action on opposite sides of the tubular part forcing the tubular part toward the die cavity surfaces while smoothly gradually mechanically forming the tubular part, as need be, to the die cavity surfaces to prevent pinching of the part between the dies as the dies are finally pressed together at their mating surfaces to form the die cavity about the part.
The die fingers and slots of the present invention thus eliminate any need for prefilling or preforming the cross-section of the part to prevent pinching as well as provide precise die alignment without added guide pins and bushings. Moreover, the die fingers and slots, which thus provide the dies with self-alignment, can be utilized to mechanically form desired bends in the part as well as provide mechanical straightening and/or substantial reshaping of the cross-section of the part prior to hydroforming.
The ability of the hydroforming dies of the present invention to perform mechanical forming of a part prior to hydroforming the part is very advantageous from both cost and manufacturing standpoints as it can eliminate the need for costly one-of-a-kind preforming press dies and the accompanying additional processing steps and/or having to prefill the part in a preliminary step. And there are, of course, limits to this ability from a mechanical metal working standpoint in order to prevent splitting or cracking of the part. But moreover, it has been found that there is also a practical limit with respect to preventing pinching of the part between hydroforming dies where the part is also being mechanically formed by the hydroforming dies as in the present invention. And in particular, it has been found that there is a practical limit in the cross-sectional relationship between the die cavity and the part as received by the fingered and slotted dies of the present invention and beyond which misshaping and/or pinching is likely to occur. For example, in the case of round tubular stock from which hydroformed parts are generally made, it was found that that portion(s) of the die cavity where mechanical forming is to take place should have a width dimension not less than about 0.8 times the initial outer diameter of the part as pinching was not found to occur at this and greater values but was found to occur at lesser values and with the likelihood of pinching progressively increasing with progressively lesser values below about 0.8.
It is therefore an object of the present invention to provide new and improved hydroforming dies.
Another object is to provide a hydroforming die set wherein the dies self-align and prevent pinching of a part as the dies close on the part.
Another object is to provide a hydroforming die set wherein the dies self-align, prevent pinching of a part as the dies close on the part, and are adaptable to bend, straighten and/or reshape the cross-section of the part during die closure.